The identification of nonsense mutations in oncogenic viruses and their analysis using suppressor cells in tissue culture would provide a powerful method to study the molecular basis of cancer. A combined biochemical and genetic approach is planned to isolate mammalian cells which contain suppressor tRNA species that enable them to translate nonsense mutations (premature polypeptide chain termination signals) and to support the growth of virus nonsense mutants. A cloned Chinese hamster cell line will be used initially although the studies will eventually be extended to include mouse, monkey, and human cell lines. Chinese hamster glutamine auxotroph mutants will be examined and nonsense mutations in the gene for the enzyme Glutamine Synthetase will be identified by the absence of cross reacting material to antibody made against purified Chinese hamster Glutamine Synthetase. Revertants are potential suppressor cells. Single-base change mutagens will be used to select mutant in the gene for hyposanthine-guanine phosphoribosyl transferase (HGPRT). Extracts of HGPRT mutant cells will be examined by SDS-acrylamide gel electrophoresis. The absence of the band corresponding to HGPRT and the appearance of a new band at a lower molecular weight will suggest a chain-termination mutation. Tryptic digest will be employed to prove that the new band is derived from the HGPRT band. Revertants of these mutants are potential suppressor cell lines containing suppressor tRNAs.